It is frequently necessary to effect a seal in a clearance gap between two components that are capable of relative movement. In particular one or more seals are often required to seal between a rotatable shaft and a static component containing the shaft, the static structure having a bore through which the shaft extends. A gas turbine engine has shafts which rotate at relatively high speeds and which are exposed to pressurised hot gases. Seals are required between the shafts and a surrounding static structure. Also a gas turbine engine has rotor discs, or drums, which rotate at relatively high speeds and which carry the rotor blades. Seals are required between the radial tips of the rotor blades and a surrounding static structure and seals are required between the rotor disc, or drum, and the static structure. The seal performance may have a significant effect on the overall efficiency of the gas turbine engine.
In gas turbine engines it is known to control the clearance between the rotor blade tips and the static structure by using materials with different coefficients of thermal expansion, by controlled heating or cooling the static structure surrounding the rotor blades, by the controlled supply of pressurised gas to a chamber to move the static structure or by using abradable seals.
In gas turbine engines it is known to use seals between the rotor disc, or drum, and the static structure for example labyrinth seals, abradable seals etc.
In gas turbine engines it is known to use seals between the shaft and the static structure for example labyrinth seals, brush seals, leaf seals and air riding seals etc.
Labyrinth seals, abradable seals, leaf seals and brush seals are passive seals whereas the controlled heating and the controlled pressurising of the static structure are active seals.
In most gas turbine engines labyrinth seals are used and these are designed to avoid large clearances at certain operating conditions, however their sealing is not as good as required and hence the specific fuel consumption is not good. The active seals have the potential to give increased sealing with a resultant improvement in the specific fuel consumption of the gas turbine engine but they are not generally used because they are complex systems.
Any improvement in the seals may improve the specific fuel consumption of the gas turbine engine.